1. Field of the Invention
The present invention relates to a quartz crystal unit, and more particularly, to a crystal unit having an excellent vibration characteristic and small dispersion in quality.
2. Description of the Related Arts
A surface mount crystal unit is widely used for an oscillator and the like as a reference source of frequency or time, especially in portable electronic devices since it is small and light. While miniaturization of a surface mount crystal unit is advancing more and more in recent years, the vibration characteristic of a crystal unit is required to be kept constant despite the miniaturization.
A conventional surface mount crystal unit shown in FIGS. 1A and 1B is realized by accommodating quartz crystal blank 2 in casing 1 made of laminated ceramics or the like, covering the opening of casing 1 with cover 3 and thereby hermetically sealing crystal blank 2 in casing 1. Casing 1 is suitable for surface mounting and provided with a recess having a substantially rectangular shape as a two-dimensional projected shape. A pair of connection terminals 4 used for an electrical connection with crystal blank 2 is provided at both sides of one end of the inner bottom surface of the recess. Connection terminals 4 are electrically connected to external terminals 5 provided on the outer surface of casing 1 via a conductive path (not shown) through the laminated plane of the laminated ceramics.
As shown in FIG. 2, crystal blank 2 has a substantially rectangular planar shape, and linear or curved inclined surfaces are formed on both principal surfaces of the crystal blank at both ends in the longitudinal direction of crystal blank 2. That is, both principal surfaces have so-called beveled surfaces or convex surfaces at both longitudinal ends of crystal blank 2. Excitation electrodes 6 are formed on the flat areas of both principal surfaces of crystal blank 2 and extension electrodes 7 are drawn out from excitation electrodes 6 toward both sides of one end of crystal blank 2 across the surface of the inclined surface at the one end. Extension electrodes 7 are folded over the principal surface on the opposite side at the position of the end face of crystal blank 2. Both sides of one end of crystal blank 2 to which extension electrodes 7 are extended are fixed onto the pair of connection terminals 4 through a conductive material, for example, conductive adhesive 8, and then crystal blank 2 is electrically and mechanically connected to casing 1 and held in the recess of casing 1. Then, by placing cover 3 so as to cover the recess, crystal blank 2 is hermetically sealed in casing 1. A crystal unit with inclined surfaces formed at both ends of a crystal blank on both principal surfaces and the crystal blank fixed to a casing by means of a conductive adhesive is disclosed in Japanese Patent Application Laid-Open No. 2001-196886 (JP, P2001-196886A).
Here, the process of fixing crystal blank 2 will be explained. As shown in FIG. 3, conductive adhesive 8 is normally applied to connection terminals 4 and then both sides of one end of crystal blank 2 on which the inclined surface is formed are positioned under a pressure indicated by arrow P. Then the assembly of casing 1 and crystal blank 2 is heated. Crystal blank 2 is thus secured through heating and hardening of conductive adhesive 8.
In the crystal unit manufactured through such a process, because of the inclined surfaces provided at both ends of crystal blank 2, vibration energy is trapped in the flat area in the center of crystal blank 2 and the vibration characteristic represented by crystal impedance (CI), etc., is thereby maintained satisfactorily.
However, the surface mount crystal unit in the above described conventional configuration has the following problems because the inclined surfaces of the crystal blank are fixed by means of conductive adhesive 8. That is, since both sides at one end of the inclined surface is pressed and then heated, the pressing force and pressing direction of the pressure applied to conductive adhesive 8 vary from one crystal blank to another depending on factors such as the positioning accuracy of the crystal blank. With such variations in the pressing force and pressing direction, the holding state and holding system also vary from one crystal blank 2 to another, causing variations in the holding state and thereby producing also non-uniformity in the vibration characteristic as the crystal unit. Furthermore, conductive adhesive 8 may wrap around from the inclined surface at the end of crystal blank 2 up to the flat area which is the vibration area and may deteriorate the vibration characteristic of the crystal unit as a consequence. These problems become noticeable because the influence of holding crystal blank 2 increases as the miniaturization of crystal blank 2 advances.
In the case of a crystal unit in a configuration with a crystal blank inserted between clamp type metal holding members, the configuration with inclined surfaces provided at both ends of one or both principal surfaces of the crystal blank is disclosed, for example, in Japanese Utility Model Application Laid-Open No. 61-70425 (JP, 61-70425, U).